An example of a video graphics system of the aforesaid type is described in British Patent No. 2,089,625 and corresponding U.S. Pat. No. 4,514,818 assigned to the present assignee. This system includes a touch tablet and stylus combination for generating position signals designating the position of the stylus relative to the touch tablet. The user paints or draws (hereinafter referred to as "painting") by selecting a desired color and implement from a range of predefined colors and implements and manipulating the stylus on the touch tablet. This manipulation causes the touch tablet to generate a series of position signals which define the path or positioning of the stylus. An image is held in a store and when a position signal is produced a new patch of pixels are derived by a processing circuit for every picture point of the image covered by the selected implement. The image data in the store is read repeatedly and is applied to a TV-type color monitor so that the build up of the picture can be observed. Video graphics systems of this type are manufactured by the present assignee and sold under the trade marks "PAINTBOX" and "GRAPHIC PAINTBOX".
In addition to painting and picture composition as described in the aforesaid patents, systems of this type can also provide for automatic processing of stored images to produce special effects. Some of these effects may be applied to the whole picture, such as a color wash or tint, but others are used much more selectively. Thus parts of an image may be required to be blurred or the reverse process (that is to say a sharpening or crisping of the image) may instead be required and such effects may be achieved by any suitable known blur or crisp algorithm. One known procedure for generating such effects involves an automatic manipulation of data for the whole image, usually involving spatial filtering algorithms or adjustment of color parameters, and results in a manipulated version of the whole of the original image being produced. The final processed data is obtained by processing the manipulated data selectively in combination with the original data by painting selected portions of the manipulated image into the original image, and thus manipulated data is only incorporated into the final image in selected regions. The painting of selected portions can be achieved by deriving pixels by combining the original image pixels with manipulated image pixels and writing the resulting modified pixels back into the framestore as described in British Patent No. 2,113,950 and U.S. Pat. No. 4,602,286 assigned to the present assignee. In this system first and second images are stored in respective first and second storing means and are combined on a picture point by picture point basis by a interpolator under the control of control image data stored in a third storing means. The control image can be created by the operator when the system is in a painting mode as described in the abovementioned British Patent No. 2,089,625 and corresponding U.S. Pat. No. 4,514,818. The control image can be used as a mask or stencil to blank off areas in the first, i.e the original, image to prevent the creation of effects in those blanked off areas of the image. Image data in the blanked off areas of the first image can be prevented from being combined with corresponding image data in the second, i.e modified, image and in this way areas in the first image in which effects are not required can be defined.
The original graphics system of this type operated with a definition suitable for standard (NTSC or PAL) television broadcasting. The abovementioned "GRAPHIC PAINTBOX" is a second generation machine arranged to operate at higher definitions than that of the original system, but recently there has been a demand for machines capable of operating at even higher definitions.
The definition of a picture is increased by increasing the number of picture elements used in defining the picture and increasing the definition of a picture also increases the time taken to copy and manipulate these elements. Thus a machine capable of operating at higher definitions would be correspondingly slower in use than a machine operating at say broadcast definition to such a degree to be irritating to the operator.